Just added to the sidebar on the right, are Moss & Co. Timber: A timber merchants in Hammersmith with reasonable pricing and a student discount. Their website is also very useful for starting out in researching different types of timber, as there are photographs of the timber's colouring, and grain, a description of the types of crafting process best suited to each timber, and its general applications.
They have a comprehensive catalogue which can be downloaded, and they also have a price list indicative of what kind of price you will need to be paying for their wares. The staff are very friendly and it is good practice to take a cutting list and/or drawing with you to the yard as they will be interested in the project you are undertaking, and how best they can suit your needs.